1. Field of the Invention
The invention relates to a digital transmission system comprising a transmitter coupled to a receiver by a channel, the receiver comprising deriving means for deriving a detection signal from an input signal received from the channel, a detector for determining detected symbols by comparing the detected signal with at least one reference value, the receiver also including adapting means for adapting the ratio between the amplitude of the detected signal and the reference value to minimize the symbol error rate.
The invention likewise relates to a receiver for such a transmission system.
2. Description of the Related Art
A transmission system as defined in the opening paragraph is known from the journal article "A PRML System for Digital Magnetic Recording" in IEEE Journal on selected Areas in Communications Vol. 10, No. 1, January 1992.
Transmission systems of this type may be used, for example, for transferring digital symbols through the public telephone network, or for reconstructing digital symbols originating from a magnetic tape or disc. When digital source symbols are transmitted via a transmission medium or when data symbols are stored on a recording medium, the digital symbols to be transmitted or recorded, respectively, are convened to analog pulses which are subsequently applied to the transmission medium or recording medium, respectively, further to be denoted by the term of channel.
The input signal of the receiver coming from the channel is convened in the receiver to a detection signal of which it is possible to determine, by a detector which value of a transmitted symbol belongs to the associated value of the detection signal. For the conversion of the input signal to a detection signal, processes such as demodulation, equalization, echo cancelling and amplitude control may be necessary. For reconstructing the transmitted symbols on the basis of the detection signal, it is necessary to compare the current detection signal, reduced, as required, by an estimate of the contributions of the preceding symbols to the detection signal, with one or more reference values. Depending on the relative value of the detection signal with respect to the reference value(s) at a detection instant, a specific symbol value is assigned to the detection signal.
Since the signal loss on the transmission path may present a strong fluctuation, the amplitude of the input signal may differ strongly for different situations. The receiver, however, is to be capable of correctly linking the value of the detection signal and the associated value of the detected symbol, despite the strongly fluctuating amplitude. In the prior-art transmission system, the comparison between detection signal and reference value is made in a Viterbi detector, but it is likewise conceivable to determine the relationship between the detection signal and values of the detected symbols by means of a simple comparator. In a number of situations, the reference values are equal to the average of two successive possible values of the detection signal (supposed to be noise-free here). With such a selection of the reference values; the symbol error rate is minimized for an equal probability of all the symbols.
Since the amplitude of the signal coming from the channel is not known a priori, the optimum value of the reference values cannot be known a priori either and the relationship between the detection signal and reference values is to be determined on the basis of the signal coming from the channel. The adaptation of the relationship between the amplitude of the detection signal and the reference values may be effected in various ways. One option is to determine the amplitude of the detection signal at the desired value with fixed reference values with the aid of a circuit for automatic gain control, further to be referenced AGC amplifier (Automatic Gain Control). An alternative solution is to adapt the reference value(s) to the amplitude of the detection signal.
Said journal article has disclosed that the adaptation of the ratio between the amplitude of the detection signal and the reference value(s) can be effected by making an estimate of the nominal value of the detection signal on the basis of the reference values and a decision about the value of a detected symbol. If the real value of the detection signal is greater than the estimate, the ratio between the amplitude of the detection signal and the reference value is adapted by increasing the reference value or by reducing the amplitude of the detection signal. If the real value of the detection signal is smaller than the estimate, the ratio between the amplitude of the detection signal and the reference value is adapted by reducing the reference value or by increasing the amplitude of the detection signal.
Experiments and simulations have shown that when the ratio between the amplitude of the detection signal and the reference values is adapted in said manner by the adapting means, it may happen that this ratio is adapted to an erroneous value which leads to a very large symbol error rate.